Clearing Disk Space and Movies not deleting

So, I edit videos in iMovie to upload to Youtube and when I go to share it to Youtube, I always get a message like "We can't upload this, your disk is full, try cleaning it up". So I go to delete a ton of videos, pdfs, etc from my computer (put them in trash and hit empty) and it doesn't help.

So, following a tutorial on Youtube, I went to my Macintosh HD and searched for movies and found the same movies I had already deleted off of my computer (so I thought) and then I use command + delete on them and they're in the trash. But when I try to empy trash it says the movies are in use and can't be deleted.

So, I downloaded a program to let me look at exactly what's taking up all my space and of the 112 GB on my Macintosh, 48 or some such is Quicktime (the movies). It also says there is space left on my disk even though iMovie doesn't think so >.>

How do I actually delete these movies/stop them being "in use" and is there a way to make less space be taken up by a video (like 1.5 GB original, sometimes there's a copy of it for no reason, and an iMovie version of like 2 GB)?

1. Buy an external drive and start "off-loading" files from your internal until you have about 20% free space, then

2. Use a disk "defragmenter/optimizer" to clean up your hard drive. This will re-arrange all your files into one contiguous grouping, and create a large area of free space "behind" the files so that a large file (such as a "1.5gb original") can be saved in contiguous format.

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